1. Field of the Invention
The invention pertains to the immobilization and remediation of metals in the soil and aqueous media using mesoporous nanocomposites.
2. Description of the Related Art
Heavy metal contamination in the soil represents a serious environmental problem that requires innovative solutions. One of the sources of heavy metal contamination arises from metal-containing ordinance used in firing ranges.
The Environmental Protection Agency (EPA) has estimated 4% or 72,575 metric tons per year of all lead made in the United States is made into bullets. Much of this lead finds it way into one of 2,600 military small arms firing ranges (SAFRs) or 9,000 non-military outdoor shooting ranges in the United States. The Department of Defense ranges typically train with the M-16, M-60 and M-9 soldier held weapons. Each 5.56 mm M193 bullet used by the M-16 rifle, contains 2.49 g of lead. The 7.62 mm M80 bullet used by the M-60 machine gun contains 6.28 g of lead and the 9.00 mm M882 ball military handgun contains 6.54 g of lead. Ranges typically use earth backstops, where lead accumulates and bullet metals, lead, copper, zinc, antimony, and potentially arsenic interacts with the soil complex and water.
Immobilization of lead (Pb) contaminates in soil and stormwater at small arms firing ranges (SAFR) is necessary to comply with environmental regulations and to protect the environment. Although metallic Pb from unweathered bullets in soil has low chemical reactivity, it is possible to mobilize Pb in the soil and aqueous media with low pH (acidity), significant changes in ionic strength or changes in the reduction oxidation potential.
Metallic Pb from unweathered bullets in soil has low chemical reactivity, but Pb can be mobilized by low pH, significant changes in ionic strength, or changes in the reduction-oxidation potential (redox), by binding metal ions to soil organic matter or held on inorganic soil through adsorption or ion exchange. Tilling or disturbing the soil has also been shown to increase the weathering of lead shot and to increase the mobility of lead in soils. Labile Pb represents a significant regulatory, environmental and health concern. Methods to reduce, control or eliminate the impact of soluble lead discharge from SAFR berms to the environment are varied and complex.
Removal of Pb2+ from contaminated soils and wastewaters by activated carbon have been reported. Precipitation, ion exchange, organic, and inorganic adsorption on activated carbon have been effective. Zeolites have also been considered as Pb2+ and other metal ion adsorbents.
Natural materials can provide lead removal efficiencies as high as 99% for clinoptilolite and chabazite. Wastewater contaminated Pb2+ and other heavy metals have been treated by 13X molecular sieves and removed >95% of the metals with a saturation adsorption time of 10 minutes. Molecular sieves 13X have been reported as effective adsorbents of Pb2+ contaminated wastewaters.
These methods utilized to remediate heavy metal contamination include four fundamental processes: physical separation, erosion control, soil modification and phyto-remediation. Physical separation techniques have been used by the chemical and mining industry for many years. The other methods have also been used under varying situations and conditions. Generally these methods are expensive and time consuming.
Nanomaterials are now coming under scrutiny in a wide variety of industrial applications. Sorptive organic-silicate materials are known as organic intermediates or precursors of a class of inorganic silicate catalysts or catalyst supports used, for instance, in petroleum refinery syntheses. However, the potential for use as environmental remediation media has not been realized because of the high calcining temperatures associated with these nanomaterials.
Accordingly, there is a desire for new technologies that can effectively remediate heavy metal contamination in the soil and aqueous systems.